Gatty Marine Laboratory, St. Andrews. 179 
it is terminated by a subulate process of some length. The 
pinne are short, and slightly tapered from base to apex. 
A white band of great purity runs round the base of the 
branchiz, bordered by a belt of red on each side. The 
filaments are barred alternately with white and red. 
Methylated spirit alters the red colour of the branchial 
plumes to a permanent blue. De St. Joseph found Tricho- 
dina pediculus, Ely., on the brauchiz at Dinard. 
The single operculum springs from the dorsal edge of the 
left fan by a somewhat rounded stem, enlarges and flattens 
out as it proceeds upward, and dilates at the summit into a 
large flat cushion for the support of the calcareous oper- 
calum, which is prettily marked with white—shield-fashion. 
Just before the last-mentioned dilatation it sends out a 
process on each side resembling an ear, and cccasionally 
this auricular process is bifid or quadrifid. The calcareous 
tip assumes various forms—viz., conical, tuberculated, 
saucer-shaped, bifid, or very often trifid. On removing the 
calcareous cap, a tough layer is left on the summit of the 
cushion with projections corresponding to the form of 
the cap. The cone is not regular, but has its steepest side 
piaced dorsally. After the action of hydrochloric acid, 
a brownish scale of organic tissue with somewhat regular 
hexagonal reticulations is left. Internaily the opercular 
edicle has a strong elastic ligament from the summit to the 
base surrounded by muscular bundles, chiefly transverse. 
The thrusting out of the operculum (if such happens) is 
thus voluntary, the withdrawal and retention more or less 
involuntary. 
The cuticle of the entire opercular apparatus is tough and 
glistening, faintly marked under the microscope with fine 
strie. The colour of the operculum varies, most being 
pale, the stalk often presenting two specks of brown. 
Almost every example from Lochmaddy, North Uist, has an 
operculum with three prongs, the only exception being oue 
here and there with only two prongs, but in all probability 
a third would by-and-by appear. A variety with a flat 
operculum from which the three spikes arose was also 
occasionally met with. The operculum is a favourite site 
for parasitic growths, such as the chambered Foraminifera, 
Vorticella, and zoophytes. 
The body is widest anteriorly, where the alar membrane 
further increases the breadth, and tapers gradually to the 
tail, which ends in a somewhat broad tip with a papilla at 
each side. It is rounded dorsally, slightly flattened and 
12* 
